Method of making low-density nonwoven fabrics

ABSTRACT

Lofty, low-density nonwoven fabrics are produced by forming a homogeneous and intimate blend of mixed thermoplastic heatretractable and non-heat-retractable fibers, forming the blend into a fibrous fleece, bonding the fibers of the fleece together at a set of discrete and spaced-apart areas by heat and pressure, and heating the bonded fabric at a temperature sufficient to cause the unaffected segments of the heat retractable fibers to retract.

Unite States Patent 1 I [111 3,898,31 1

Mitchell et al. Aug. 5, 1975 METHOD OF MAKING LOW-DENSITY 3,236,238 2/1966 Morse 264/342 NONWOVEN FABRICS 3,507,943 4/1970 Such et a1. 264/103 Inventors: Philip A. Mitchell, East Walpole;

John ,1. Such, Wrentham, both of Mass.

Assignee: The Kendall C0,, Walpole, Mass.

Filed: Nov. 8, 1971 Appl. No.: 196,336

Related US. Application Data Continuation-impart of Ser. No. 844,549, July 24,

1969, abandoned.

US. Cl. 264/103; 264/122; 264/126;

264/342; 264/DIG. 71 Int. Cl. B29C 25/00 Field of Search 156/85, 433; 264/D1G. 71,

References Cited UNITED STATES PATENTS Russell et a1 156/291 Primary Examiner-Robert F, White Assistant Examiner-T. E. Balhoff 5 7] ABSTRACT Lofty, low-density nonwoven fabrics are produced by forming a homogeneous and intimate blend of mixed thermoplastic heat-retractable and non-heatretractable fibers, forming the blend into a fibrous fleece, bonding the fibers of the fleece together at a set of discrete and spaced-apart areas by heat and pressure, and heating the bonded fabric at a temperature sufficient to cause the unaffected segments of the heat retractable fibers to retract.

2 Claims, 3 Drawing Figures PATENTEU AUE 51975 METHOD OF MAKING LOW-DENSITY NONWOVEN FABRICS This application is a continuation-in-part of Ser. No. 844,549, filed July 24, 1969, now abandoned.

This invention relates to a method for improving the loft and thickness of nonwoven fabrics. More specifically, it relates to a method of producing spot-bonded nonwoven fabrics of exceptionally low density and absorptive power, suited for use as disposable dusters, wiping cloths, blankets, and the like.

Nonwoven fabrics of various types are finding rapidly increasing usage as dusting, cleaning, and polishing cloths, as fluid or paste applicators, and for a variety of allied uses. In such applications they are generally classified as disposable or limited use items, and are preferred over woven fabrics for reasons of convenience and economy. The majority of such fabrics are composed of textile-length cellulosic fibers such as rayon or cotton fibers, or mixtures thereof, carded or garnetted into a fleece and then bonded with an overall or a patterned application of a polymeric binder.

Such prior art products, however, are not ideal in all respects. In their behavior when folded or crumpled in use they are apt to be paper-like, lacking in the easy conformability of woven or knitted fabrics. Furthermore, the inter-fiber spacing is generally of small dimensions, the fibers being closer together and in' a more compacted configuration than is desirable. Close fiber-to-fiber contact promotes high capillarity in an absorbent fibrous article, but decreases the potential total absorbency or pick-up. Particularly when dust or particulate matter is to be picked up by a disposable nonwoven fabric, a porous fabric of low density, with substantial spaces or pores between fibers, is preferred both for efficiency in pick-up and in case of shaking or washing the fabric clean after use.

Also, the insulating value of conventional nonwoven fabrics leaves something to be desired, since the close inter-fiber spacing minimizes the dead air space essential to proper insulation required for limited-use blankets, sleeping bag liners, and the like.

It is the primary object of this invention to provide nonwoven fabrics of exceptionally low density, suitable for use as disposable dusters, all-purpose cleaning cloths, blankets, and the like.

As starting material in the practice of this invention we employ a substantially homogeneous fleece of intermingled textilelength non-thermally-sensitive fibers containing intimately blended therewith a proportion of fibers which are heat-retractable and thermoplastic. By textile-length is meant those fibers, usually averaging one-half inch in length or longer, which can be dryassembled into a fleece or web by conventional textile fiber-separation means such as cards, garnetts, air-lay machines, and the like. By heat-retractable is meant those fibers which, due to their chemical nature or processing history, tend to contract in length when heated to temperatures below their melting point. Typical heat-retractable fibers comprise the polyolefin fibers polyethylene and polypropylene; polyvinyl chloride and polyvinyl chloride-polyvinyl acetate copolymers; undrawn polyester and polyamide fibers; plasticized cellulose acetate, and the like. While the proportion of heat-retractable fibers in the blended fleece may be varied to suit the particular properties desired in the final product, in general the heat-retractable fibers constitute between 10% and 50% of the total weight of fibers, with a preferred range of 20% to 30%.

The non-thermally-sensitive fibers may be any textile fibers which are unaffected at the temperatures at which the thermoplastic heat-retractable fibers are affected. For absorbency and economy, cellulosic fibers such as rayon or cotton are preferred.

The blended fibrous fleece is then bonded by passing it between a pair of rolls engraved in a pattern of lands and grooves, as set forth in U.S. Pat. No. 3,507,943, of common assignee, incorporated herein by reference. At least one of the rolls is heated, and the result is a spot-bonded product, as discussed more fully hereinbelow.

For the subsequent development of maximum loft and low density in the end product, it is preferred that the bonded areas constitute not over 30% of the total surface area of the fleece and that the bonded areas be separate, discrete, and isolated from each other by areas of fiber which are unbonded or lightly bonded.

The thus bonded nonwoven fabric is then heated to a temperature sufficient to cause the unfused segments of the heat-retractable fibers to contract in length, but not to melt, while the fabric is allowed to shrink. In general, an area change of 10% to 30% will take place in this process, with a disproportionate volume change and an unexpectedly large decrease in the density of the product, of from 40% to The invention may be better understood by reference to the following description and drawings, in which FIG. 1 is a highly-enlarged cross-sectional view of a typical spot-bonded nonwoven fabric before treatment.

FIG. 2 is a cross-sectional view of the product of FIG. 1 after shrinkage by heat-treatment.

FIG. 3 is a schematic representation of the distribution of bonded and unbonded areas in a spot-bonded fabric suitable for use in the process of this invention.

As explained in U.S. Pat. No. 3,507,943, the process of passing a fleece of intermingled thermoretractile and thermoplastic fibers blended with non-thermally sensitive fibers between heated rolls engraved in a pattern of lands and grooves results in a bonded fabric wherein there are spaced-apart quadrilateral areas of fused and intermingled fibers of both types, as shown at 30 in FIG. 3 and at 10 in FIGS. 1 and 2. Such bonded areas result where a land on one roll has traversed a land on the other roll in the calendering process, and represent areas of maximum compaction. They are bounded along their sides by quadrilateral areas where a land has traversed a groove, as at 32 in FIG. 3, and at their apices by quadrilateral areas where a groove has traversed a groove. The resulting product, FIG. 1, is prior art, set forth in U.S. Pat. Nos. 3,507,943 and 3,542,634.

Microscopic examination of the prior art fabric of FIG. 1 shows that in the areas of maximum" compression, both types of fiber are fused together into a quadrilaterally shaped spot which remains dimensionally stable up to the melting point of the thermoplastic fibers. In the other quadrilateral areas where a land has traversed a groove, however, and particularly where a groove has traversed a groove, there lie segments of thermoplastic fibers which are either very lightly fused or which are substantially unaffected, so that they are still capable of being thermally retracted when heated to temperatures below their melting point. As set forth above, such a secondary heating process causes an area shrinkage of 10% to 30%, with a decrease in density of from 40% to 60%, andthe prior art fabric of FIG. 1 is converted to the low-density product of FIG. 2.

The basis of the present invention, therefore, lies in subjecting an intermingled fleece of heat-retractable thermoplastic fibers and non-thermally sensitive fibers to sufficient heat and pressure so as to fuse the two types of fibers together-in a set of discrete and spacedapart quadrilateral areas while leaving segments of the thermally-sensitive fibers lying between the fused areas substantially unaffected, and then heating the bonded product in a secondary heating operation to a temperature sufficient to cause the substantially unaffected segments of the thermally-sensitive fibers to retract but not to melt, said secondary heating being conducted while the fabric is free to expand in thickness while decreasing in area.

The invention will be illustrated by the following examples, produced by the process of U.S. Pat. No. 3,507,943, but using rolls with different sizes and spacings of helical lands. The composition of the fibrous fleece was in each case 75% l-9/l6 inch 1.5 denier viscose rayon and either 25% 1.5 inch 1.8 denier polypropylene fibers or 25% undrawn polyester fibers, the fibers being an intimately blended mixture and not stratified or layered in different proportions. In each instance the top roll was heated to about 450F. and the lower roll to about 440F., and the pressure was 100 pounds per inch of nip. Also in each instance, the spotbonded pattern produced was in the shape of rhomboidally-shaped bonded areas as shown in FIG. 3, arranged in diagonal rows, the dark spots 30 indicating areas where a land on one roll traversed a land on the other roll with sufficient heat and pressure to fuse the polypropylene fibers to the rayon fibers in a set of discrete and spaced-apart bonded areas, the stippled areas 32 indicating areas of intermediate compaction where a land on one roll traversed a groove on the other roll, and the unshaded areas 34 indicating a substantially I uncompacted area where a groove on one roll traversed a groove on the other roll.

EXAMPLE 1 Using a carded fleece of 75% rayon 25% polypropylene weighing 48 grams per square yard and the general embossing process set forth above, the rayon and polypropylene fibers were fused together in a set of diamond-shaped bonded areas, the area of each bonding spot being 0.001 1 square inches. There were 196 spots per square inch, with lateral spacing of 0.086 inches and longitudinal spacing of 0.103 inches. The bonded spots occupied 22% of the total area of the fabric.

The thickness of the fabric was 0.022 inches, as measured by an Ames Gauge Model No. 1202 with a pressor foot 1.5 inches in diameter, used in all thickness tests in this description.

The product in general resembled the fabric shown in FIG. 1, with rhomboidal areas 10 wherein a land traversing a land had heat-fused the rayon fibers and the thermoplastic fibers. These areas 10 were bounded by diagonal ridges of lightly-compressed or uncompressed fibers 14.

Samples of the above bonded product were then heated to 350F. for 1 minute in a circulating air oven. The thickness of the product increased to 0.030 inches, or by 36%. Area shrinkage was 7%, the volume increase of the fabric was 27%, and the density of the fabric, in grams per cubic centimeter, decreased from 0.114 original value to 0.090, or 21% decrease.

The product resembled that of FIG. 2, wherein the rhomboidal areas 10 of maximum pressure remain unchanged, so that the thickness 12 of these areas 10 is unaffected by the heat treatment, while the diagonally intersecting ribs 14 have increased in thickness substantially.

EXAMPLE 2 A similar fleece, weighing 48 grams per square yard was hot-embossed with a set of diamond-shaped bonded areas, the area of each bonding spot being 0.0039 square inches. There were 64 spots per square inch, with lateral spacing of 0. 1 87 inches and longitudinal spacing of 0.157 inches. The bonded spots occupied 23% of the area of the fabric. The thickness of the fabric was 0.022 inches.

EXAMPLE 3 A fleece of similar composition but weighing 67 grams per square yard was hot-embossed with a set of diamond-shaped bonded areas, the area of each bonding spot being 0.011 square inches. There were 25 spots per square inch, with lateral spacing of 0.369 inches and longitudinal spacing of 0.239 inches. The bonded spots occupied 25% of the area of the fabric. The thickness of the fabric was 0.033 inches.

Samples of the above bonded fabric were then heated to 350F. for 1 minute in a circulating air oven. The thickness of the product increased to 0.109 inches, an increase of 230%. The area shrinkage was 30%, the volume increase was 130%, and the density of the fabric had decreased from an original value of 0.095 grams per cubic centimeter to 0.041 grams per cubic centimeter, .a decrease of 57%.

EXAMPLE 4 A carded fleece of rayon fibers and 25% undrawn, heat-retractable polyester fibers was embossed as in Example 2. The bonded fabric weighed 84 grams per square yard.

Samples of the bonded fabric were heated at 360F. for 1 minute in a circulating air oven. The thickness of the product was found to haveincreased from .031 inches to .051 inches, an increase of 65%. Area shrinkage was 14%, the volume increase of the fabric was 35%, and the density had decreased from 0.146 grams per cubic centimeter to 0.108 grams per cubic centimeter, a decrease of 26%.

1 Having thus described our invention, we claim:

1. The method of producing a lofty, low-density nonwoven fabric characterized by criss-crossing diagonally intersecting ribs which comprises assembling a substantially homogeneous and intimately blended mixture of thermoplastic heatretractable fibers and nonthermally sensitive fibers, said thermoplastic heat-retractable fibers constituting between and 50% of the total weight of said homogeneous mixture;

forming said fibers into a fibrous fleece; subjecting said fleece to heat and pressure at a set of discrete and spaced apart quadrilaterally-shaped areas, said heat and pressure being sufficient to fuse the two types of fibers together in said quadrilaterallyshaped areas. while leaving the segments of fibers lying between said quadrilaterally-shaped areas substantially unaffected; and

subjecting said fleece to a second heating process,

thermally sensitive fibers are cellulosic fibers. 

1. THE METHOD OF PRODUCING A LOFTY, LOW-DENSITY NONWOVEN FABRIC CHARACTERIZED BY CRISS-CROSSING DIAGONALLY INTERSECTING RIBS WHICH COMPRISES ASSEMBLING A SUBSTANTIALLY HOMOGENEOUS AND INTIMATELY BLENDED MIXTURE OF THERMOPLASTIC HEAT-RETRACTABLE FIBERS AND NONTHERMALLY SENSITIVE FIBERS, SAID THERMOPLASTIC HEAT-RETRACTABLE FIBERS CONSTITUTING BETWEEN 10% AND 50% OF THE TOTAL WEIGHT OF SAID HOMOGENEOUS MIXTURE, FORMING SAID FIBERS INTO A FIBROUS FLEECE, SUBJECTING SAID FLEECE TO HEAT AND PRESSURE AT A SET OF DISCRETE AND SPACED APART QUADRILATERALLY-SHAPED AREAS, SAID HEAT AND PRESSURE BEING SUFFICIENT TO FUSE THE TWO TYPES OF FIBERS TOGETHER IN SAID QUADRILATERALLY-SHAPED AREAS, WHILE LEAVING THE SEGMENTS OF FIBERS LYING BETWEEN SAID QUADRILATERALLY-SHAPED AREAS SUBSTANTIALLY UNAFFECTED, AND SUBJECTING SAID FLEECE TO A SECOND HEATING PROCESS, SAID SECOND HEATING PROCESS BEING CONDUCTED IN THE ABSENCE OF PRESSURE AND AREA RESTRAINT, AND BEING OF SUFFICIENT INTENSITY AS TO CAUSE ALL OF SAID PREVIOUSLY UNAFFECTED SEGMENTS OF THE THERMALLY-SENSITIVE FIBERS TO RETRACT BUT NOT TO MELT.
 2. The method according to claim 1 wherein said thermoplastic heat-retractable fibers constitute between 20% and 30% of the total weight of said homogeneous mixture and are polypropylene, and said non-thermally sensitive fibers are cellulosic fibers. 